Furnace for continuous firing of phosphors

ABSTRACT

A furnace for continuous firing and recovery of fluorescent material including a rotating burner which generates a flame into which the fluorescent material is introduced. The fluorescent material is transported by gaseous stream into a chamber where it is mixed with a fuel, and the mixture is then forced through rotating nozzles, located at the top of the furnace, where the fuel burns and fires the fluorescent material.

United States Patent Martha J. B. Thomas Winchester; Ernest A. Dale,Hamilton; William A. Flnch, Mlrblehead, all 01 Mass. Appl. No. 258 FiledJan. 2, 1970 Patented Sept. 21, 1971 Assignee Sylvania Electric ProductsInc.

[72] inventors FURNACE FOR CONTINUOUS FIRING 0F PHOSPIIORS 4 Claims, 3Drawing Figs.

US. Cl 23/277 R, 431/182, 431/185, 252/30l.4, 239/406, 23/284 Int. CL.....F27b 15/10,

[50] Field ofSearch 23/277, 252, 284; 252/3014; 75/21 1, 84.5; 266/34.I, 34.2; ll7/l05.2,9l.5; ll8/47;431/4, 9,185,182;

[56] References Cited UNITED STATES PATENTS 836,219 11/1906 Schultz431/9 Primary Examiner-James I-I. Tayman, Jr. AnorneysNorman J. O'Malleyand Owen J. Meegan S s s FURNACE FOR CONTINUOUS FIRING OF PI-IOSPIIORSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to furnaces and particularly to those which can beused for the manufacture of phosphors. Continuous firing of materialscan be provided through the use of the present furnace.

2. Description Of The Prior Art US. Fat. to Butler, No. 2,755,254discloses a method of making a halophosphates phosphor in a continuousmanner in a furnace. The raw materials necessary to form the phosphorwere blended in the proper proportions and then placed in open silicaboats. These boats were then placed in an elongated furnace and moved ina semicontinuous manner form one end of the furnace to the other. Ablanket of inert gas flowed through the furnace in a direction oppositeto the direction in which the boats were moved.

In a US. Pat. to Homer et al., No. 3,093,594, an improvement on themethod of firing is disclosed in which the furnace was divided intozones. Inert gas flowed counter to the direction of the boats in thefirst zone and then in the same direction in the second zone. In a thirdzone the inert gas then flowed in a direction counter to the directionin which the boats were traveling. In the last zone, the gas flowed inthe same direction as the boats. Between each of the zones, interlockswere provided which allow different firing temperatures and changes inthe direction of flow of the inert gas.

These furnaces have been the core of the processing equipment for fullscale production of fluorescent materials. They have worked well andprovided a superior phosphor which has been used in the manufacture offluorescent lamps.

Many other improvements have been made in the processing techniqueswhich were used during the firing. For example, the US. Pat. to Homer,No. 3,002,933, discloses an important technique to prevent traces ofmanganese from boiling out of the raw material blend and depositing onthe surface of the blend as a pink-top." Through the relativelyuncomplicated step of not filling the boats to the brim, thevolatilization of the manganese is inhibited.

Other techniques and modifications have been made in the semicontinuousfiring approach discussed above but the fundamental concept of firinghas not been changed. For example, the US. Pat. to Vodoklys, No.3,023,339 discloses a post firing treatment of a phosphor in which thephosphor was slurried in water and dispersed through an atomizer intoheated gas. According to patentee, the treatment can improve thefinished phosphor through the elimination of milling steps.

DESCRIPTION OF THE DRAWING FIG. 1 is an elevational cross-sectional viewof the furnace for preparing phosphors according to the presentinvention.

FIG. 2 is a cross-sectional view of the burner assembly which isdisposed on the top of the furnace.

FIG. 3 is a view of the lower surface, taken along the line 33 of theburner assembly shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thefurnace includes a lining 1, preferably formed of a refractory materialin an elongated tubular shape with the upper end and lower end 3 and 5,respectively, converging upon the axis of the tube.

Surrounding the lining is a refractory insulator 8, preferably in theform of blocks, to retain the heat within the furnace. The thickness ofthe insulator 8 should be sufficient to prevent the formation of anylocalized cold spots.

A burner assembly is disposed at the upper end of the furnace and theflow path is directed along the axis of the lining. Details of theburner are more specifically shown in FIG. 2. Raw materials for theprocess can be formed as disclosed in the application of Dale et al.,Ser. No. 606,159, filed Dec. 30, I966, now abandoned, and continuationapplication Ser. No. 56,l63 filed July 8, 1970, entitled Process OfForming Phosphors and assigned to the same assignee as the presentinvention.

These phosphor-forming materials are transferred, preferably by air,from a central conduit 7 into the interior of the furnace. Combustiblegas, natural gas for example, is introduced through conduit 9 into anannular mixing chamber 11 wherein the gas and the phosphor areintermixed. A secondary supply of combustion supporting gas, preferablyair, is introduced through conduit 10 and enters into casing 15 whichserves as a manifold to supply secondary air to the nozzles 17. Nozzles17 are preferably an'anged about the annular mixing chamber 11 androtate to facilitate mixing of the burning gases and thephosphor-forming particles which are passing therethrough.

A tube 19 of ceramic material surrounds the nozzles 17 to promoteturbulent flow of the gas and powder suspended therein and therefore,insure mixing. Such turbulence also reduces the likelihood ofagglomeration of the particles. Buildup of particles upon the walls ofthe ceramic tube 19 is also inhibited by the rotation of nozzles 17.

The specific details of the burner assembly are shown in FIG. 2. Theparticles of phosphor-forming material are transported by air throughconduit 7 which terminates below the entrance port 21 for the naturalgas from conduit 9. The phosphor and air mix with the natural gas inannular mixing chamber 11 and vent through port 23 into tube 19.

Annular mixing chamber 11 is divided into two parts, the

upper part of which is fixedly disposed upon the burner. Bolts 24 attachto annular cover plate 25. Disposed beneath the upper part is a lowerpart which is formed by tube 31. Worm drive gear 27 is attached to amotor (not shown) and mating worm drive gear 29 is attached to tube 33to rotate concentric conduit 33. Tube 31 remains stationary. Attached tothe lower end of conduit 33 is an annular flange 35 through which aplurality (preferably eight) of holes are formed. The peripheral edge ofthe annular flange 35 rests upon a fixed ledge 37 which provides supportfor the lower part of the annular mixing chamber and also prevents airfrom leaking into the furnace. Nozzles 17 are radially disposed in theannular flange and will rotate at the desired speed (r.p.m.) whichconduit 33 rotates. Peripheral cavity 18 provides a space for the flowof cooling media around flange 37 to prevent overheating. Bearings 28provide support for the rotation of conduit 33 around tube 31.

Secondary air from conduit 10 enters into casing 15 which forms amanifold for the distribution of air through nozzles 17 and thence intothe furnace. The ends of the nozzles 17 in the furnace should be beneaththe mouth of the tube 31 which forms the annular mixing chamber so as toprovide a flame front which begins wholly within tube 19.

As seen in FIG. 3, the nozzles are radially disposed about annularflange 35 which in turn surrounds mixing chamber 11. A refractory heatshield 39 with peripheral recesses, cut out to accommodate the nozzles17 is attached to the bottom of flange 35. Surrounding the periphery ofthe burner is tube 19.

Referring again to FIG. 1, the lower portion 5 of the furnace is reducedin diameter to form an exit outlet 2 which terminates in a water zone 4.Cooling water for the manufactured phosphor is introduced through spraynonle 6. A suspension of phosphor and water is withdrawn from water zone4 for drying and elimination of the water.

In operation, the particles of powder are introduced into the upper endof the furnace through the innermost tube 7, the diameter of which isthree-fourths inch, at a rate of 40 lbs/hour with a transport gas (air)rate of 625 cubic feet/hour. Natural gas is introduced through theintermediate tube 9, the diameter of which is one and five-eighthsinches. Secondary air is introduced through the conduit 10 and flowsinto the manifold formed by casing 15. The air is introduced intoconduit 10 at the rate of 2,000 cubic feet/hour. From the manifold, theair flows through eight nozzles 17 which extend three inches below theannular mixing chamber 11. With the three inch extension, together withthe close-proximity of the powder and natural gas inlets, the phosphormixes thoroughly with the gases before entering the furnace and beforebeing effected by a flame front which begins within the tube 19. Theflame can be initiated by an electric arc, such as a spark plug, whichis disposed within the tube 19.

The flame front starts slightly below the nozzles 19 and extendsdownward about 2 to 3 feet, filling substantially the entirecross-sectional area of the chamber, thereby effecting substantially allthe particles in the path downward through the chamber.

A heat resistant ceramic tube 19 surrounds the nozzles 17 and is 5inches in diameter and 9 inches long. The tube promotes turbulence inthe gas stream and enhances mixing and prevents agglomeration of theindividual particles. The furnace is a ceramic cylinder 1, 16 inches indiameter and 16 feet high.

In order to further improve the mixing of the natural gas and air andfor improving uniformity in the flame temperature, the nozzles arerotated at a rate of 10 rpm. This rotation also helps prevent stickingand building of hot particles on the walls of the hot ceramic tube.

As the converted particles drop to the bottom of the chamber, they arecollected in a container 4 of water in order to separate them from theexhaust gases. In addition, the exhaust gases are passed through acyclone separator (not shown) to recover particles carried out with thegases. The collected particles are then centrifuged to separate themfrom the water and subsequently dried to yield a free flowing powder.

It is apparent that modifications and changes may be made within thescope of the instant invention. it is our intention however only to belimited by the scope of the appended.

claims.

As our invention, we claim:

1. A furnace for continuous manufacture of phosphors comprising: anelongated tubular furnace having an enlarged central portion; a burnerdisposed at the upper end of said furnace, said burner being adapted toempty the products thereof into said furnace; said burner includingfeeding means disposed within an outer casing and adapted to feed agaseous suspension of unfired phosphor into the furnace; means tointroduce a fuel into a mixing tube concentrically disposed about thelength of said feeding means and extending therebeyond, whereby the fuelwill mix with the gaseous suspension of unfired phosphor; a plurality ofnozzle means radially disposed about said mixing tube; the egress ofsaid nozzles being disposed beneath the egress of said mixing tube; theegress of said nozzles being disposed beneath the egress of said mixingtube; a second tube disposed about the nozzles, said second tube beingcoaxially disposed about said mixing tube and extending into the body ofsaid furnace; a manifold disposed in gas flow relationship with saidnozzles and adapted to serve as a supply of secondary air for thecombustible gases in said mixing tube; means to rotate said nozzleswhereby tur bulence is produced in the mixture of gases and phosphor;means to remove the phosphor from said furnace.

2. The furnace according to claim 1 wherein said casing serves as saidmanifold for the supply of secondary gas into said nozzles.

3. The furnace according to claim 1 wherein the means to rotate thenozzles includes a conduit which is disposed about the mixing tube andwhich is attached to a flange in which the nozzles are disposed, saidflange resting upon a lip which is disposed upon the outer casing.

4. The furnace according to claim 3 wherein a heat shield is disposedbeneath said flange to insulate the flange from the heat in the furnace.

2. The furnace according to claim 1 wherein said casing serves as saidmanifold for the supply of secondary gas into said nozzles.
 3. Thefurnace according to claim 1 wherein the means to rotate the nozzlesincludes a conduit which is disposed about the mixing tube and which isattached to a flange in which the nozzles are disposed, said flangeresting upon a lip which is disposed upon the outer casing.
 4. Thefurnace according to claim 3 wherein a heat shield is disposed beneathsaid flange to insulate the flange from the heat in the furnace.